You need to use the IUXConfiguration.RegisterSite(...) method to first add a UXSiteSetup where UXSiteType property is set as UXSiteType.RibbonGroup to the approriate context’s ribbon UXSite and then add your command into that new group.
What UXSites exist by default and how they relate to each other is maybe easiest to understand by looking at how they are configured in the VisualComponents.Engine.exe.config file in the VC installation folder.